1. Field of the Invention
This invention relates to communications systems employing speech detectors; more particularly, this invention relates to a system for increasing speech detection accuracy by suitably evaluating the variations with time of the envelope associated with a given voice channel signal.
2. DESCRIPTION OF THE PRIOR ART
The problem of increasing the traffic capacity of telephone lines or links in telephone communication systems has been continually increasing due to increases in both demand and cost of such transmission. There have been several prior art attempts to optimize such telephone transmission, such as by what has been commonly termed speech interpolation. However, such speech interpolation systems depend, for maximum efficiency, on correctly deciding whether or not, at a given instant, a speech signal is present in a given telephone voice channel signal. Furthermore, the quality or reliability of such telephonic transmission is directly dependent on the correct detection of the presence or absence of the speech signal in all of the telephone voice channels associated with a single such speech interpolation system. Such speech detection, however, is made both difficult and costly by the presence of background noise, having nothing to do with the speech signal, on each of the associated telephone voice channels, such noise normally being present in telephonic transmissions. In order to compensate for the presence of such noise, prior art speech detection systems operate by evaluating the envelope of a given telephone voice channel signal and then comparing this evaluation with a preset constant threshold level which is preset at a value higher than the noise level normally found in such voice channel signals. Therefore, if the envelope, also known as the shorttime-power, exceeds this threshold level, such prior art systems presume that useful speech information is present and the channel thereby assumes an active state (active channel) in which the voice channel signal will be transmitted. However, if the evaluated envelope is less than this preset threshold level, such prior art systems presume that only noise is present in which case the channel thereby assumes an idle state (idle channel) in which the voice channel signal is not transmitted. Thus, these prior art systems associate the presence of speech with a relatively high power threshold and vice versa. Such prior art speech detection is not satisfactory, however, since the speech envelope may at various times be less than the preset threshold level in which instance the system would not transmit information which should have been transmitted, thereby diminishing the quality and reliability of such prior art systems.
These disadvantages of the prior art are overcome by the present invention.